<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>For循环优化</title>
</head>
<body>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <script>
       var aBtns = document.getElementsByClassName('btn')
       for(var i = 0; i< aBtns.length; i++) {
           console.log(i)
       }
       for(var i = 0, len = aBtns.length; i< len; i++) {
           console.log(i)
       }
    </script>
</body>
</html>